1. Field of the Invention
The present invention relates to the field of data networking and, in particular, to a method and apparatus for preserving flow order across links of an multi-link trunk (MLT).
2. Background Information
As computer technology has evolved, so too has the use of networks which communicatively couple computer systems together allowing remote computer systems to communicate with one another. The improved computer technology, along with the widely distributed nature of corporate computing and the cost/accessibility of high bandwidth data networks has fostered the growth of multi-media network applications over such networks. One example of just such a network topology is the Ethernet standard topology. In recent years, we have seen the Ethernet standard evolve from a 10 Mb/S standard to a 100 Mb/S standard as we race towards the 1 Gb/S standard. Although the prospect of gigabit Ethernet technology will reduce much of the congestion experienced on current Ethernet LAN implementations, those skilled in the art recognize that the additional bandwidth will quickly be consumed by bandwidth-hungry multimedia applications. Thus, another approach is required to improve the bandwidth efficiency of such networks.
One approach currently being considered is the use of multiple physical data links to facilitate the transmission of information, a method commonly referred to as link aggregation. Those skilled in the art will appreciate that link aggregation is a technique which permits one to treat multiple physical links as one logical link, also commonly referred to as a multiple link trunk (MLT). Link aggregation is the topic of study for the Institute for Electrical and Electronic Engineers (IEEE) 802.3ad study group, which is working to define protocols for the exchange of traffic over multi-link trunks. One of the objectives of the study group is maintaining the ordering of frames. In many network protocols receiving frames out of order is likely to cause confusion. Indeed, the ramifications of processing out of order frames are often unpredictable and thus, undesirable. Similarly, the receipt of duplicate frames can also cause problems in many communication protocols. The typical solution to having received an out-of-order and/or duplicate frame sequence is the retransmission of the entire frame sequence. Given a no-contention network architecture such as, for example, the Ethernet network wherein only one network element may be actively transmitting at a time, the need to retransmit entire frame sequences significantly reduces network efficiency.
To improve the efficiency of such networks, a number of solutions are currently being considered to preserve frame ordering across aggregated links, the so-called multi-link trunk. To date, proposed solutions focus on the transmit side of the communication. One proposed solution, for example, relies on tagging frames with sequence numbers at the transmit side, and removing the sequence numbers from the frames as the frames are received and promoted. Although this method is currently favored in the technical community as providing an easy resolution of the problem, those skilled in the art recognize that such a solution is a costly one insofar as it involves altering the frame structure. That is, instead of simply routing frames a network bridge or switch, for example, must modify the frames to add the sequence numbers, thereby violating a number of bridging protocols. By violating such bridging protocols, a problem of backward compatibility is created, leaving legacy bridges that are unable of supporting aggregated link communication sessions.
Another problem commonly associated with prior art aggregated link control techniques arises on the transmit side when handling “flows”, i.e., a sequence of messages or frames that have the same source, destination and quality of service requirements. Prior art switches identify a flow and queue the frames identified as a flow on a single, particular link. Those skilled in the art will appreciate that queuing a flow through a single link, as done in the prior art, eliminates many of the benefits commonly associated with use of an aggregated link, e.g., maximizing throughput, load balancing, etc. due to the management required to switch the entire flow to another physical link.
Thus a method and apparatus for preserving frame ordering across aggregated links between source and destination nodes is required that does not resort to modification of the frames themselves. Accordingly, a method and apparatus for preserving frame ordering across aggregated links is presented which is unencumbered by the inherent deficiencies and limitations commonly associated with the prior art.